U.S. patent number 5,899,752 [Application Number 08/429,432] was granted by the patent office on 1999-05-04 for method for in-situ cleaning of native oxide from silicon surfaces.
This patent grant is currently assigned to Applied Materials, Inc.. Invention is credited to David Carlson, H. Peter W Hey.
United States Patent |
5,899,752 |
Hey , et al. |
May 4, 1999 |
Method for in-situ cleaning of native oxide from silicon
surfaces
Abstract
A method of in-situ cleaning a native oxide layer from the
surface of a silicon wafer positioned in a vacuum chamber that is
substantially free of oxidizing species by passing at least one
non-oxidizing gas over the native oxide layer at a wafer cleaning
temperature between about 650.degree. C. to about 1025.degree. C.
for a sufficient length of time until such native oxide layer is
removed.
Inventors: |
Hey; H. Peter W (San Jose,
CA), Carlson; David (Santa Clara, CA) |
Assignee: |
Applied Materials, Inc. (Santa
Clara, CA)
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Family
ID: |
22284982 |
Appl.
No.: |
08/429,432 |
Filed: |
April 26, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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101502 |
Jul 30, 1993 |
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Current U.S.
Class: |
438/791;
257/E21.226; 438/906; 257/E21.293; 257/E21.212 |
Current CPC
Class: |
H01L
21/02046 (20130101); H01L 21/3003 (20130101); H01L
21/3185 (20130101); Y10S 438/906 (20130101) |
Current International
Class: |
H01L
21/318 (20060101); H01L 21/02 (20060101); H01L
21/306 (20060101); H01L 21/30 (20060101); H01L
021/318 () |
Field of
Search: |
;438/906,775,791 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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A 0430030 A2 |
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Jun 1991 |
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EP |
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3-22527 |
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Jan 1991 |
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JP |
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Other References
SWolf Ph.D. et al., "Silicon Processing for the VLSI Era vol. 1:
process technology," Lattice Press, CA, 1986, pp. 166-184. .
S.Mukherjee, "Advanced Processing of Semiconductor Devices,"
Proceedings of SPIE Mar. 23-25, 1987 vol. 797, 10 pp. 90-97. .
"Investigation of Thermal Removal of Native Oxide . . . " by
Tatsuya Yamazaki, et al., 1046 Journal of the Electrochemical
Society 139 (1992) Apr., #4, Manchester, NH/US pp. 1175-1180. .
"Effect of Silicon Surface Cleaning . . . " by K. Saito, et al.,
2419 Japanese Journal of Applied Physics (1989) Aug. 28-30, 21th
Conf., Tokyo, Japan, pp. 541-542. .
"Removing Native Oxide from Si (001) Surfaces . . . " by Takayuki
Aoyama, et al., Applied Physics Letters 59 (1991) Nov., 11, No. 20,
New York, US, pp. 2576-2578. .
"Si Surface Cleaning by Si.sub.2 H.sub.6 -H.sub.2 Gas Etching . . .
" by Yasuo Kunii, et al., Japanese Journal of Applied Physics, vol.
26, No. 11, Nov., 1987, pp. 1816-1822. .
"Alternate Surface Cleaning Approaches . . . " by M. Racanelli, et
al., 1046 Journal of the Electrochemical Society 138 (1991) Dec.,
No. 12, Manchester, NH, US, pp. 3783-3789. .
"Letter to the Editor LPCVD Si.sub.3 N.sub.4 Growth Retardation on
Silicon . . . " Francois Martin, et al., 8303 Semiconductor Science
& Technology 6 (1991) Nov., #11, Bristol, GB pp.
1100-1102..
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Primary Examiner: Bowers; Charles
Assistant Examiner: Whipple; Matthew
Attorney, Agent or Firm: Fish & Richardson P.C.
Parent Case Text
This is a continuation-in-part of copending application Ser. No.
08/101,502 filed on Jul. 30, 1993.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of processing a silicon wafer, the wafer having a
native oxide thereon, comprising:
positioning said silicon wafer in a chemical vapor deposition
chamber;
evacuating said chamber until the chamber pressure is less than
about 10.sup.-5 Torr of oxygen and water;
flowing hydrogen and at least one other reducing gas selected from
the group consisting of silane and dichlorosilane over the surface
of said silicon wafer;
thereafter, heating said silicon wafer to a wafer cleaning
temperature of not higher than about 1025.degree. C. while
continuing to flow hydrogen over the surface of said silicon wafer
until an evaporation and a chemical reaction remove substantially
all native oxide from the surface of the silicon wafer; and
then forming a layer of silicon nitride on the surface of said
silicon wafer without removing the wafer from said chamber by
passing a mixture of dichlorosilane gas and ammonia gas over the
surface of said silicon wafer at a deposition temperature not
higher than said wafer cleaning temperature.
2. A method according to claim 1 further comprising, after the step
of removing the native oxide, reducing the temperature within said
chamber to a deposition temperature in the range between about
700.degree. C. to 800.degree. C.
3. A method according to claim 1 wherein said native oxide is
removed within no more than 1 minute.
4. A method according to claim 1 wherein said wafer cleaning
temperature is in the range between 700.degree. C. to 840.degree.
C.
5. A method according to claim 1, wherein said wafer cleaning
temperature is preferably in the range between about 800.degree. C.
to about 1025.degree. C.
Description
FIELD OF THE INVENTION
The present invention generally relates to a novel method of
cleaning native oxide from a silicon wafer surface and more
particularly, relates to a novel method of in-situ cleaning a
native oxide layer from the surface of a silicon wafer by heat in
the presence of at least one reducing gas.
BACKGROUND OF THE INVENTION
In the fabrication of semiconductor devices on silicon wafers,
various structures such as metalization layers, passivation layers,
insulation layers, etc. are formed on a silicon substrate. The
quality of the semiconductor device fabricated is a strong function
of the processes with which these structures are formed. The
quality is also a function of the cleanliness of the manufacturing
environment in which the silicon wafer is processed.
Technological advances in recent years in the increasing
miniaturization of semiconductor circuits require more stringent
control of impurities and contaminants in the processing chamber of
the semiconductor device. When the miniaturization of the device
progressed to the submicron level, the minutest amount of
contaminants can significantly reduce the yield of wafers.
Among the electronic materials frequently used for deposition on
silicon wafers, silicon nitride has gained more importance in
recent years. Silicon nitride is used extensively as a final
protective passivation and coating layer for semiconductor wafers
because of its excellent diffusion barrier characteristics against
moisture and alkali ions. Silicon nitride is a desirable
semiconductor material also for its high density and high
dielectric properties.
The deposition of silicon nitride is usually performed by a low
pressure chemical vapor deposition (hereinafter LPCVD) process in
which dichlorosilane and ammonia are reacted together in a heated
chamber. The reaction between silicon tetrachloride or
dichlorosilane and ammonia normally takes place with nitrogen
carrier gas at approximately 650.degree. C. or with hydrogen
carrier gas at approximately 1,000.degree. C.
In a LPCVD process for the deposition of silicon nitride, the LPCVD
chamber is first evacuated, a mixture of gases of silicon
tetrachloride or dichlorosilane and ammonia is then introduced into
the chamber which contains one or more silicon wafers each having a
surface onto which a silicon nitride layer is to be deposited. The
silicon wafers are generally heated to a deposition temperature at
the time when the mixture of gases are fed into the chamber such
that the gases decompose and thereby depositing a silicon nitride
layer on the surface of the wafer. For instance, in a prior-art
LPCVD system equipped with a horizontal boat that receives a
plurality of silicon wafers positioned vertically in the chamber,
reactant gases are injected into the chamber through a number of
apertures and flow across the wafers. The use of such prior-art
chambers encourages the growth of native oxide on the surfaces of
the wafers.
Chemical native oxide is not a true silicon dioxide because it is
not stoichiometrically formed. Native oxide layers are typically
formed after a cleaning procedure partially due to the presence of
moisture in the air. Native oxide is chemically different than
grown silicon oxide which is intentionally deposited or formed. The
physical properties of native oxide and silicon dioxide are also
different, for instance, the refractive index of silicon dioxide is
typically 1.45 while the refractive index for native oxide is
approximately 2.2.
Native oxide also forms on wafer surfaces during prior processing
steps which expose the wafer to ambient conditions. Some
semiconductor processes include various cleaning steps prior to
deposition of electronic materials. However, the cleaned wafers are
usually still exposed to the ambient atmosphere after such cleaning
and native oxide has the opportunity to grow on the wafer surface
prior to the silicon nitride deposition.
For instance, the handling of multiple wafers in a wafer boat at
multiple processing stations during a semiconductor fabrication
process causes particular problems with regard to the formation of
native oxide. The wafers take significant amount of time to load
into the chamber, i.e. on the order of thirty minutes. During such
loading step, air is present around the wafers and a native oxide
layer readily forms on the newly cleaned surface of the wafer. This
problem is compounded by the fact that such native oxide formation
is not uniform, i.e., the first wafer in the chamber may grow a
thicker layer of native oxide. This leads to a batch of integrated
circuit structures having different electrical properties depending
on the particular wafer from which the integrated circuit
structures were formed.
It is therefore desirable that prior to the deposition of any layer
of semiconductor materials on a silicon wafer, the surface of the
wafer should be clean and free of contaminants such as native oxide
or other impurities. Contaminants present at the interface between
the silicon surface of the wafer and the layer formed thereon
interfere with the electrical properties of the integrated circuit
structures resulting in degraded performance or total failure of
the structure.
It has been observed that the growth of silicon nitride films on a
silicon wafer can be affected by the presence of native oxide on
the silicon surface. This manifests itself as an "incubation time"
during which growth of the nitride layer is retarded on the native
oxide surface. A typical test for this incubation time can be
conducted by depositing layers of silicon nitride under identical
conditions but with varying deposition times. Longer deposition
times correspond to thicker nitride films. A graph of the silicon
nitride film thickness versus the deposition time therefore shows a
straight line. This is shown in FIG. 1 as the solid line. The slope
of this straight line represents the growth rate of the silicon
nitride films.
Theoretically, the intercept at time equal to zero should show that
the thickness of the nitride film is zero. However, in reality,
thin films of silicon nitride exhibit a variable intersect of the
horizontal axis at times from one to thirty or more seconds. The
length of this time is referred to as the "incubation time", shown
in FIG. 1 as A, since little growth of silicon nitride is observed
during this time period. We have discovered that various process
parameters affect the length of incubation time, as well as the
thickness of native oxide on the silicon surface.
The incubation time has several detrimental effects in the
formation of silicon nitride films on semiconductor devices. For
instance, thin layers of silicon nitride are frequently used to
form capacitor structures. The thickness control for thin silicon
nitride dielectric films is therefore very surface and process
dependent which in turn makes the capacitor control very difficult.
Furthermore, a native oxide layer of variable thickness can reduce
the capacitance of the dielectric layer, i.e. native oxide plus
nitride, and further degrade the capacitor performance.
Others have attempted to clean a native oxide layer from the
surface of a silicon wafer by a baking process at temperatures as
high as 1,200.degree. C. However, such high temperature baking is
only suitable in an epitaxial silicon growth process and not suited
for a silicon nitride film process which is normally deposited at a
much lower temperature.
It is therefore an object of the present invention to provide a
novel method of in-situ cleaning native oxide from the surface of a
silicon wafer that does not have the shortcomings of the
conventional cleaning method.
It is another object of the present invention to provide a novel
method of in-situ cleaning native oxide from the surface of a
silicon wafer such that a clean interface is provided between a
single crystal silicon wafer and a subsequently deposited
semiconductor material.
It is a further object of the present invention to provide a novel
method of in-situ cleaning native oxide from the surface of a
silicon wafer such that the incubation period normally encountered
in a silicon nitride film deposition process can be eliminated.
It is another further object of the present invention to provide a
novel method of in-situ cleaning native oxide from the surface of a
silicon wafer by heating the wafer to a modest temperature in an
environment substantially free of oxidizing species such as oxygen
and water.
It is yet another further object of the present invention to
provide a novel method of in-situ cleaning native oxide from a
silicon wafer surface by heating the wafer to a modest temperature
in an environment substantially free of oxidizing species in the
presence of at least one reducing gas.
SUMMARY OF THE INVENTION
In accordance with the present invention, a novel method of in-situ
cleaning a native oxide layer from the surface of a silicon wafer
in a substantially oxygen and water free environment by the
combined processes of physical evaporation and chemical reaction is
provided.
In the preferred embodiment, the novel in-situ cleaning method is
provided by heating a silicon wafer to a temperature in the range
of 800.about.1025.degree. C. while concurrently flowing a hydrogen
or other reducing gas at a sufficiently high flow rate in an
environment substantially free of oxidizing species such as oxygen
and water. The necessary conditions for native oxide layer removal
in this preferred embodiment are an environment that is
substantially free of oxidizing species in a leak-tight process
chamber and a modest chamber temperature. The hydrogen gas is used
to provide the necessary background level of reducing or
reoxidizing species. A native oxide layer is typically removed in
one minute or less under these conditions.
In an alternate embodiment, the novel in-situ cleaning method is
provided by adding a reactive gas to the process chamber to
facilitate the native oxide removal. A precursor for silicon CVD
such as silane, dichlorosilane at very low partial pressures can be
utilized to grow a non-continuous silicon layer above the native
oxide layer to facilitate the removal of native oxide at
approximately 700.about.900.degree. C.
In another alternate embodiment, the novel in-situ cleaning method
is provided by adding to the process chamber a reactive gas of
germane which removes native oxide at approximately
700.about.800.degree. C. through the formation of volatile
germanium-oxygen compounds. Other alternate reactive gases such as
fluorine-containing species are also used to chemically attack the
native oxide and cause its removal.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present invention
will become apparent upon consideration of the specification and
the appended drawings, in which:
FIG. 1 is a graph showing the silicon nitride film thickness as a
function of the deposition time.
FIG. 2 is a cross-sectional view of a typical single-wafer CVD
apparatus.
FIG. 3 is a flow diagram illustrating the novel in-situ cleaning
process in combination with a deposition process for silicon
nitride films.
FIG. 4 is a graph showing the silicon nitride film thickness as a
function of the deposition time measured at the center and at the
edge of a wafer.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The present invention discloses a novel method of in-situ cleaning
of a native oxide layer from the surface of a silicon wafer by
heating the wafer to a modest temperature in an environment that is
substantially free of oxidizing species such as oxygen and
water.
Referring initially to FIG. 2, where a cross-sectional view of a
typical single-wafer CVD apparatus is shown. Such single-wafer
processing equipment are being designed as multi-chamber clustered
integrated processing system incorporating the use of load-lock
systems wherein a wafer can be transported from one single-wafer
process chamber to another through a central load-lock system
without breaking vacuum. One such system for CVD deposition is
supplied by the Applied Materials Corporation in Santa Clara,
Calif. under the trademark of Centura.TM. HT Poly.
A thermal reactor 10 for processing a silicon wafer 40 that has a
housing 12, a double-dome reactor vessel 14 that defines a reactor
chamber 16, a gas inlet manifold 18, a gas exhaust manifold 26, a
heating system 20, a drive assembly 22, a susceptor 24, and a
preheat ring 28, is shown in FIG. 2. Double-dome vessel 14 includes
a top dome 30 and a bottom dome 32 which are cooled by circulating
cooling air such that a cold wall, i.e. at 100.about.200.degree. C.
is maintained. The drive assembly 22 is coupled to a motor (not
shown) to rotate susceptor 24 during the deposition process to
enhance coating uniformity.
The present invention provides a novel method of in-situ cleaning
native oxide from the surface of a silicon wafer such that the
incubation period observed in a silicon nitride film deposition
process can be eliminated. This novel method can best be understood
by an examination of FIG. 3, which shows a flow chart of the
present method.
As shown in FIG. 3, the present method can be practiced by first
loading a wafer into a typical cold-wall CVD deposition system. The
wafer can be either a silicon wafer or a silicon wafer with a
silicon dioxide coating. These wafers may have been previously
cleaned by dipping in hydrogen fluoride. The load-lock chamber is
first evacuated to a pressure of less than 10 Torr. The wafer is
then loaded from the load-lock chamber into the process chamber at
the deposition pressure. The temperature of the chamber is then
raised to 800.about.950.degree. C. while flowing with hydrogen gas.
When hydrogen gas alone is used for cleaning the native oxide from
a silicon surface, it is flowed at a rate up to 20 SLM while the
chamber is heated to 900.degree. C. The evacuation process insures
that there are substantially no oxidizing species, i.e. less than a
partial pressure of 10.sup.-5 Torr of oxygen and water left in the
chamber. The native oxide layer can be cleaned in approximately one
minute.
The partial pressure of oxygen and water is measured by a typical
residual gas analyzer technique used for vacuum systems. One of
such analyzer utilized in the present work is the Inficon Quadrex
200, Model #901-002-G1, which is manufactured by the Leybold
Inficon Co. of East Syracuse, N.Y.
As shown further in FIG. 3, the cleaning step may also comprise the
step of flowing the chamber with a reactive gas together with the
hydrogen. This reactive gas can be either silane or dichlorosilane
flowed at a low flow rate. The purpose of adding silane or
dichlorosilane to the chamber is to grow a non-continuous silicon
layer over the native oxide layer such that the native oxide can be
removed at about 800.degree. C. It is noted that by the addition of
silane or dichlorosilane, a lower cleaning temperature of
800.degree. C., instead of 900.degree. C. when hydrogen gas alone
is used, can be utilized. Other silicon-bearing gases may also work
as reactive cleaning gases for removing native oxide from silicon
surfaces. The flow rate used for the silicon-bearing gas is
normally less than 1 sccm which is substantially lower than the
flow rate used for hydrogen.
After the cleaning step, as shown in FIG. 3, the chamber
temperature is lowered to 750.+-.50.degree. C. while continuously
purging with hydrogen. It should be noted that while FIG. 3 shows a
continuous process of first cleaning and then depositing a silicon
nitride film on the same silicon wafer, the cleaning step
illustrated in FIG. 3 can be used for other deposition processes.
For instance, it can be used prior to a selective epitaxial growth
(SEG) process in which epitaxial silicon is grown inside windows of
silicon nitride or polysilicon. The temperature used in a SEG
process is relatively low, i.e. 750.about.850.degree. C., when
compared to a conventional epitaxial silicon growth process which
requires a minimum temperature of 1,100.degree. C. The SEG process
is important for making advanced devices in which a planar surface
is desired which requires the windows to be filled completely by
epitaxial silicon.
When the deposition of a silicon nitride film is desired on a
cleaned silicon wafer surface, as shown in FIG. 3, an ammonia gas
may first be added to the chamber for a nitridation process. It was
discovered that such a nitridation process forms a pre-deposited
silicon nitride layer up to approximately 10 .ANG. thickness. This
is shown in FIG. 1 by the intercept of the dashed line. FIG. 1 also
shows a solid line which indicates a silicon nitride film
deposition process conducted by a conventional method without wafer
surface cleaning indicating an incubation time of approximately 20
seconds.
After the nitridation process, the deposition of silicon nitride is
continued by flowing dichlorosilane into the reactor chamber. After
a sufficient thickness of silicon nitride film is formed by the
reaction between dichlorosilane and ammonia, i.e. up to 1500 .ANG.,
the flow of ammonia and dichlorosilane is stopped and the chamber
is purged with hydrogen gas prior to the unloading of the wafer
from the process chamber.
FIG. 4 shows data of the silicon nitride film thickness as a
function of the deposition time. The film thickness is measured
both at the center of the wafer and at the edge of the wafer. Only
hydrogen gas was flowed through the reactor chamber. It is seen
that a uniform film of silicon nitride is formed on the silicon
wafer. Data shown in this graph is generated at a 900.degree. C.
bake temperature, a 775.degree. C. deposition temperature, and a
chamber pressure of 100 Torr.
Table I shows data generated in seven tests by using hydrogen and
silane gases at various temperature and pressure settings.
TABLE I ______________________________________ Clean- Flow Rate
Chamber ing Temp., H.sub.2, SiH.sub.4, Pressure, Change, Test.sup.#
Gas .degree. C. SLM sccm Torr Substrate .ANG.
______________________________________ 1. H.sub.2 900 10 0 20
Silicon- -3 No HF 2. H.sub.2 900 20 0 100 Silicon- -1 HF Dip 3.
H.sub.2 900 20 0 100 Silicon- -2 HF Dip 4. SiH.sub.4 750 10 .4 20
Oxide -4 ______________________________________
Tests #1 through #3 were conducted with a cleaning gas of hydrogen
only. When the chamber temperature is at 900.degree. C., a
reduction of the native oxide thickness is observed in all three
tests. The efficiency of native oxide film removal does not seem to
increase with the flow rate of the hydrogen gas. The cleaning
efficiency of hydrogen gas is higher for silicon wafers that were
not previously cleaned by a hydrogen fluoride dip. A suitable range
of chamber temperatures when native oxide film is cleaned by
hydrogen alone is between 800.degree. C. and 1025.degree. C.
When silane is added to the chamber with hydrogen gas, as shown in
Tests #4, it is seen that an efficient cleaning by a mixture of
hydrogen and silane is obtained at a low chamber temperature of
750.degree. C. Test #4 was conducted on a silicon dioxide surface.
It should be noted that the flow rate used for silane is several
orders of magnitude smaller than the flow rate used for hydrogen. A
suitable range of chamber temperature for cleaning by silane or
dichlorosilane is between 700.degree. C. and 840.degree. C.
The cleaning efficiency of fluorine-containing gases and
germane-containing gases has also been tested. These gases are
suitable for cleaning native oxide from the surface of a silicon
wafer at a lower cleaning temperature of approximately 700.degree.
C. to 750.degree. C. Satisfactory cleaning results have been
obtained by using these gases in cleaning native oxide from silicon
wafers.
The novel method of cleaning native oxide from surfaces of silicon
wafers has been demonstrated as an effective and advantageous
process for preparing high quality silicon wafers. Even though only
silicon nitride films and epitaxial silicon films deposited by a
selective epitaxial growth process are discussed, any other films
of semiconductor materials can be deposited on a silicon wafer
surface after such cleaning procedure.
While the present invention has been described in an illustrative
manner, it should be understood that the terminology used is
intended to be in a nature of words of description rather than of
limitation.
Furthermore, while the present invention has been described in
terms of several preferred embodiments thereof, it is to be
appreciated that those skilled in the art will readily apply these
teachings to other possible variations of the invention. For
instance, reactive gases other than silane, dichlorosilane,
germane, and fluorine can be used which may be equally effective in
achieving the desirable results of the present invention.
* * * * *